Transgenic mice overexpressing the nonmuscle myosin heavy chain-IIB (NMHC-IIB) specifically in cardiac myocytes or more ubiquitously are being established and will be crossed to the NMHC-IIB deficient mice to test if adding the NMHC-IIB back to the cardiac myocytes is sufficient to prevent the NMHC-IIB deficient mice from developing the structural cardiac defects. The results will also be useful to evaluate effects of the NMHC-IIB on the development of the nonmuscle cells, such as neuronal cells which are enriched in NMHC-IIB. The latter has been plagued by the fact that NMHC-IIB deficient mice died shortly before or after birth. Currently, the NMHC-IIB plasmid has been constructed and the transgenic mice are being produced with our collaborators. The green fluorescent protein (GFP) fused to human NMHC-IIB cDNA has also been used to trace the localization of the NMHC-IIB in transfected living cell lines. The GFP fused to a N-terminal fragment containing the head and neck domain dispersed in the cytoplasm and nucleus of the transfected COS-7 and RBL cell lines, as was also seen with GFP control construct. However, the GFP fused to the tail domain or the complete sequence of human NMHC-IIB cDNA was localized in these cells, close to the plasma membrane, or to perinuclear regions. Further dissection of the tail region showed that a small domain of the NMHC-IIB tail comprised of about 300 amino acids is necessary and sufficient for the localization of the human NMHC-IIB.